Electromagnetic field adjustment for proximity detection

ABSTRACT

Embodiments described herein relate to a methods and apparatus for optimizing and calibrating a magnetic field generator. In various embodiments, the magnetic field generator includes a signal generator for outputting a voltage and a magnetic field generating circuit including a shunt that may be used to change a value of inductance in the magnetic field generating circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/064,733, filed on Mar. 24, 2008, the subject matter of which isincorporated in its entirety by reference herein.

BACKGROUND

Various embodiments described generally relate to safety systems at worksites, and in particular to a proximity detection system and methods ofadjusting an electromagnetic filed produced by the same.

Many methods have been devised to protect people from being struck,pinched, crushed or otherwise harmed by vehicles and mobile equipmentused for above and below ground operations. Examples of the equipmentinclude: road construction equipment such as trucks, road graders,rollers and pavers; surface mining equipment, such as for use withgravel and sand operations, front end loaders, trucks, dozers, conveyorsand other items; underground mining equipment such as continuous miners,shuttle cars, conveyors, crushers, load-haul-dump vehicles, man-trips,tractors; and other equipment such as fork lifts, cranes, and trucksused at warehouses and shipping ports.

A number of different proximity detection systems have been devised toprotect people and property in these industrial operations, such as thesystems disclosed in U.S. Patent Application Publication No.2006/0087443, and U.S. Pat. Nos. 6,810,353 and 5,939,986, which areincorporated by reference herein in their entirety. These proximitydetection systems establish a warning zone around dangerous equipment orareas by generating a magnetic field perimeter. A worker who enters themagnetic field perimeter carrying a magnetic field detection device iswarned of his or her presence within the magnetic field perimeter andhis or her corresponding proximity to the dangerous vehicle. To maintaina warning zone of an appropriate size, the proximity detection systemsmay be calibrated or otherwise optimized at a factory or laboratorybefore installing the system in the field.

However, industrial environments involve a wide range of climatic andenvironmental conditions that may cause a proximity detection system toperform differently under field conditions compared to the controlledconditions in a laboratory or factory. In some cases, the size of themagnetic filed perimeter created by the proximity detection system inthe field may be significantly different from the size of the perimetercreated under ideal conditions. Therefore, there is a need to be able toadjust a proximity detection system at the time of installation, orthereafter, to optimize performance and/or to calibrate the system tothe desired operational settings.

SUMMARY

When a proximity detection system is installed or is in use in thefield, the system may need to be calibrated and/or optimized to accountfor variable conditions such as changes in cable lengths, interferenceto the magnetic field from the equipment it is mounted to, substitutionof system elements, temperature, moisture, degradation of componentsover time, and the like. Various embodiments described herein providemethods and apparatus to make adjustments to the proximity detectionsystem during or after installation.

In one embodiment described herein, a magnetic field generator includesa signal generator for outputting a voltage and a magnetic fieldgenerating circuit for generating a magnetic field. The magnetic fieldgenerating circuit includes a capacitor and an inductor comprising aferrite core. The magnetic field generator also includes a shunt movablysupported relative to the inductor such that moving the shunt changes avalue of inductance in the magnetic field generating circuit.

In another embodiment described herein, a method for adjusting theresonance of a magnetic field generator includes installing a magneticfield generator at a location at which the magnetic field generator isto be used. The magnetic field generator includes a signal generator foroutputting a voltage, a magnetic field generating circuit for generatinga magnetic field, and a shunt movably supported relative to theinductor. The magnetic field generating circuit includes a capacitor andan inductor having a ferrite core. The shunt is supported relative tothe inductor in such a way so that moving the shunt changes a value ofinductance in the magnetic field generating circuit. The method includesmoving the shunt relative to the inductor to increase the resonance ofthe magnetic field generating circuit for a given voltage.

In another embodiment described herein, a safety system includes aplurality of alarm devices for detecting a magnetic field, and amagnetic field generator. The magnetic field generator includes a signalgenerator for outputting a voltage, a capacitor in electroniccommunication with the signal generator, an inductor in electroniccommunication with the capacitor, and a ferrite core at least partiallysurrounded by the inductor. The magnetic field generator also includesand a shunt movably supported relative to the inductor in such a way sothat moving the shunt changes a value of inductance in the magneticfield generator.

In another embodiment described herein, a method for adjusting the rangeof a magnetic field generated by a magnetic field generator includesinstalling a magnetic field generator at a location at which themagnetic field generator is to be used, positioning a personal alarmdevice at a distance from the magnetic field generator, generating amagnetic field using the magnetic field generator so that the magneticfield encompasses the personal alarm device, and detecting the magneticfield with the personal alarm device. The method further includesdecreasing the magnetic field size until the personal alarm devicecannot detect the magnetic field, incrementally increasing the magneticfield size until the personal alarm device detects the magnetic field,and setting the magnetic field size equal to the magnetic field size atwhich the personal alarm device detected the magnetic field.

In another embodiment described herein, an apparatus for adjusting themagnetic field strength of a magnetic field generator includes amagnetic field generator. The magnetic field generator includes a signalgenerator for outputting a voltage and a magnetic field generatingcircuit for generating a magnetic field. The magnetic field generatingcircuit includes a capacitor and an inductor comprising a ferrite core.The magnetic field generator further includes a shunt movably supportedrelative to the inductor such that moving the shunt changes a value ofinductance in the magnetic field generating circuit, a controller forcontrolling the magnetic field generating circuit, and a voltage dividercircuit. The voltage divider circuit is positioned electrically inparallel with the inductor of the magnetic field generating circuit andoutputs a voltage to the controller. The controller inputs a voltagefrom the voltage divider circuit and uses the inputted voltage tocontrol the voltage output by the signal generator, thereby controllingthe strength of the magnetic field.

In another embodiment described herein, a method for adjusting thestrength of a magnetic field generated by a magnetic field generatorincludes installing a magnetic field generator at a location at whichthe magnetic field generator is to be used, positioning a personal alarmdevice at a distance from the magnetic field generator, and generating amagnetic field using the magnetic field generator so that the magneticfield encompasses the personal alarm device. The magnetic fieldgenerator includes a signal generator for outputting a voltage, acontroller, a capacitor, and an inductor. The method further includesdetecting the magnetic field with the personal alarm device, decreasingthe magnetic field size until the personal alarm device cannot detectthe magnetic field, incrementally increasing the magnetic field sizeuntil the personal alarm device detects the magnetic field, determiningand storing in the controller the voltage across the inductor of themagnetic field generator at which the personal alarm device firstdetects the magnetic field, and adjusting the signal generator using thecontroller to keep the voltage across the inductor at the same level aswhen the personal alert device was first detected.

In another embodiment described herein, a method for adjusting the rangeof a magnetic field generated by a magnetic field generator for aproximity detection system includes installing a magnetic fieldgenerator at a location at which the magnetic field generator is to beused, selecting a power level of the magnetic field generator less thanthe maximum power level of the magnetic field generator and generating amagnetic field by operating the magnetic field generator at the selectedpower level. The method includes adjusting the position of a shuntrelative to the magnetic field generator to maximize the magnetic fieldsize of the magnetic field generator at the selected power level,positioning a personal alarm device at a distance from the magneticfield generator, and detecting the magnetic field with the personalalarm device. If the magnetic field is not detected by the personalalarm device, the method further includes incrementally increasing themagnetic field size until the personal alarm device detects the magneticfield and setting the magnetic field size equal to the magnetic fieldsize at which the personal alarm device detected the magnetic field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an exemplary proximity detection system inaccordance with an embodiment described herein.

FIG. 2 is a diagram of an exemplary proximity detection system inaccordance with an embodiment described herein.

FIG. 3 is a diagram of an exemplary magnetic field generator of thesystem of FIG. 2 in accordance with a first embodiment described herein.

FIG. 4 is a view like FIG. 3 of a second embodiment of a magnetic fieldgenerator.

FIG. 5 is a view like FIG. 3 of a third embodiment of a magnetic fieldgenerator.

FIGS. 6 and 7 are diagrams of a portion of an exemplary magnetic fieldgenerator of the system of FIG. 2 in accordance with an embodimentdescribed herein.

FIGS. 8 and 9 are diagrams of a portion of an exemplary magnetic fieldgenerator of the system of FIG. 2 in accordance with another embodimentdescribed herein.

FIG. 10 is a diagram of a portion of an exemplary magnetic fieldgenerator of the system of FIG. 2 in accordance with another embodimentdescribed herein.

FIG. 11-15 are diagrams of an exemplary method for calibrating aproximity detection system in accordance with an embodiment describedherein.

FIGS. 16 and 17 are diagrams of an exemplary method for detectingimpending collisions using the proximity detection system in accordancewith an embodiment described herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which are shownby way of illustration specific embodiments that may be practiced. Theseembodiments are described in sufficient detail to enable those ofordinary skill in the art to make and use them, and it is to beunderstood that structural, logical, or procedural changes may be madeto the specific embodiments disclosed herein.

FIG. 1 shows an exemplary proximity detection system 101 that may beused to warn personnel if they are dangerously close to equipment or toother dangerous zones within a mine or above ground. A protection zoneis established by a magnetic field generator 102 that creates apulsating low frequency oscillating magnetic marker field 104 around theequipment 105. The magnetic marker field 104 is detected by a personalalarm device 103 worn or otherwise carried by a person 100. If theperson 100 wearing the personal alarm device 103 enters the magneticmarker field 104, the personal alarm device 103 will emit an audiblewarning and a display on the equipment 105 will provide a visual and/oraudible warning to the operator of the equipment 105. Where applicable,the system 101 may be capable of issuing a command to slow or stop theequipment 105 if the person 100 enters the magnetic marker filed 104.

To provide a protection zone of a proper size around a particular pieceof equipment 105, it may be desirable to optimize or otherwise calibratethe magnetic field generator 102 to maintain the magnetic marker field104 at a constant size. However, industrial environments involve a widerange of climatic and environmental conditions and the magnetic fieldgenerator 102 may perform differently under field conditions compared tothe controlled conditions in a laboratory or factory where the magneticfield generator 102 may have been optimized or calibrated. Therefore,various embodiments described herein provide systems and methods tooptimize and/or calibrate the proximity detection system 101 at the timeof installation, or thereafter.

FIG. 2 is a diagram of an exemplary proximity detection system 501 inaccordance with an embodiment further described below. A magnetic fieldgenerator 502 includes a signal generator 510 connected to a magneticfield generating circuit 506. Various embodiments of magnetic fieldgenerating circuits 506 are described in greater detail below and inFIGS. 3, 4, and 5. The magnetic field generating circuit 506 generates amagnetic field 504 in response to an input voltage from the signalgenerator 510. The signal generator 510 is also connected to a powersource 520 which supplies power to the magnetic field generator 502. Acontroller 515 is connected to the signal generator 510 and controls thevoltage and current outputs of the signal generator 510. The controller515 is also connected to a receiver 517 and warning system 590. Thereceiver 517 is connected to an antenna 518 which receives an inputsignal 514 from a personal alarm device 503. The antenna 518 conveys thesignal 514 to the receiver 517 which passes the signal 514 to thecontroller 515. Upon receiving the signal 514 from the personal alarmdevice 503, the controller 515 directs the warning system 590 to issue awarning. In one embodiment, the warning system 590 may issue an audioand/or visual warning. In another embodiment, the warning system 590 maybe capable of terminating the operation of a vehicle to which thewarning system 590 is mounted.

The personal alarm device 503 has x, y, and z axis magnetic fieldantennas 534 that sense the magnetic field 504 produced by the magneticfield generator 502. The sensed magnetic field signal 504 is passedthrough filters 533 and an amplifier 532 to a signal detector 531. Thesignal detector 531 then passes information about the detected signal toa controller 516. The controller 516 activates a transmitter 536 whichtransmits a corresponding response signal 514 to the magnetic field 504through a RF antenna 519. In one embodiment, the response signal 514 isan RF signal. The personal alarm device 503 is powered by power source522. The personal alarm device 503 may be carried by a worker in orderto provide the worker with a warning of their proximity to a magneticfield generator 502. In another embodiment, the personal alarm device503 may be mounted on a vehicle or other piece of equipment to which aproximity warning is sought. In another embodiment, the magnetic fieldgenerator 502 may be mounted in a location in which it is desirable towarn a worker carrying a personal alarm device 503 of their proximity.

FIG. 3 is a diagram of an exemplary magnetic field generator 202 of thesystem of FIG. 2 in accordance with a first embodiment described herein.A power source 220 powers a signal generator 210 that generates avoltage across the series circuit 206. The series circuit 206 includesan inductor 255 and a ferrite core 260. In one embodiment, the inductor255 at least partially surrounds the ferrite core 260. The seriescircuit 206 also includes a capacitor 250 connected in series with theinductor. A controller 215 is connected to and controls the operation ofthe signal generator 210. The magnetic field generator 202 includes awarning system 290, receiver 217, and antenna 218 similar to thosedescribed with regard to FIG. 2. The magnetic field generator 202 alsoincludes a movable shunt 265.

The signal generator 210 produces low frequency voltage oscillations. Inone embodiment, the oscillations of the signal generator 210 occurbetween approximately 25 kHz to 100 kHz. In another embodiment, theoscillations of the signal generator 210 may be at a frequency ofapproximately 73 kHz. The voltage oscillations produced by the signalgenerator 210 are applied to the inductor 255 through the capacitor 250and result in an oscillating magnetic field 204. The magnetic field 204produced by the magnetic field generator 202 expands and collapsesaround the magnetic field generator at a frequency equal to that ofvoltage oscillations produced by the signal generator 210.

The controller 215 controls the operation of the signal generator 210 tomanage the frequency and magnitude of the voltage oscillations producedby the signal generator 210. The controller 215 may control the size ofthe magnetic field 204 by controlling the duration of voltage pulsesproduced by the signal generator 210. For example, an increase in theduration of voltage pulses provided to the series circuit 206 by thesignal generator 210 will create a larger magnetic field 204.Conversely, shorter voltage pulses provided to the series circuit 206will create a smaller magnetic field 204. The size of the magnetic field204 may also be controlled by adjusting the magnitude of the voltageoscillations produced by the signal generator 210. For example, avoltage oscillation of higher magnitude results in a larger magneticfield 204, while a voltage oscillation of lower magnitude will result ina smaller magnetic field 204.

The size of the magnetic field 204 may also be adjusted by manipulatingthe resonance of the series circuit 206. The series circuit 206containing the inductor 255 and capacitor 250 will have a givenresonance at which charge passes between the inductor 255 and capacitor250. A charge stored on the capacitor 250 will discharge across theinductor 255. The inductor 255 creates a magnetic field 204 in whichenergy is stored as the charge passes through the inductor 255. Once thecapacitor 250 is discharged, the energy stored in the magnetic field 204by the inductor 255 begins to be reconverted into electrical energy anda charge of opposite polarity to the original charge is stored on thecapacitor 250. This cycle continues indefinitely producing the samemagnitude charge on the capacitor 250 and the same magnitude magneticfield 204 in the absence of resistance within the circuit 206. Thefrequency of the oscillations depends upon the absolute values of theinductance of the inductor 255 and capacitance of the capacitor 250. Theresonant frequency of the circuit 206 is the frequency at which theoscillations occur when the absolute values of the inductance of theinductor 255 and capacitance of the capacitor 250 are equal.

In one embodiment, the resonance of the inductor 255 and the capacitanceof the capacitor 250 may be approximately equal. For example, theinductance of the inductor 255 may be approximately 300 microHenry andthe capacitance of the capacitor 250 may be approximately 163microfarad. Other suitable values of inductance and capacitance may alsobe used, for example, to create a series circuit 206 in which a maximumcurrent is passed through the inductor 255 while using a minimumvoltage. In one embodiment, the values for the inductance of theinductor 255 and capacitance of the capacitor 250 may be chosen tomaximize the resonance of the series circuit 206.

The resonance of the series circuit 206 may be adjusted by a shunt 265positioned adjacent to or otherwise near the inductor 255. In oneembodiment, the shunt 265 is positioned next to the inductor 255. Inanother embodiment, the shunt 265 at least partially surrounds theinductor 255 and the ferrite core 260. For instance, the shunt 265 maybe on only one side of the ferrite core 260, two sides, three sides, orcompletely surround the ferrite core 260. The shunt 265 may besemi-cylindrical, cylindrical, flat with various shapes, such as square,bent at an angle, such as a 90 degree angle, or other shapes. The shunt265 may be constructed from aluminum, copper, or other suitablenon-ferric metals and alloys.

Movement of the shunt 265 in relation to the inductor 255 and ferritecore 260 changes a value of inductance of the inductor 255. The changein inductance of the inductor 255 in turn changes the resonance of theseries circuit 206. In one embodiment, the shunt 265 may be movedrelative to the inductor 255 and ferrite core 260 so as to maximize theresonance of the series circuit 206 at a given voltage produced by thesignal generator 210. For instance, if the shunt 265 is positioned nearthe center of the longitudinal axis of the ferrite core 260, theinductance of the inductor 255 is reduced a minimal amount. Theinductance of the inductor 255 decreases by a larger amount as the shunt265 is moved away from the center of the ferrite core 260 and towardseither end of the ferrite core 206. In an embodiment, the shunt 265 maybe moved anywhere along the length of the ferrite core 260. In anotherembodiment, the shunt 265 may be moved from the center of the ferritecore 260 to beyond the ends of the ferrite core 260. Furthermore, theshunt 265 may be movably supported in relation to the ferrite core 260in any suitable manner. For instance, the shunt 265 may be mounted on ahousing surrounding the ferrite core 260, upon an object at distancefrom the ferrite core 260, or in any other manner in relation to theferrite core 260.

FIG. 4 is a view like FIG. 3 of a second embodiment of a magnetic fieldgenerator. A power source 320 powers a signal generator 310 thatgenerates a voltage across an inductor 355 and a first capacitor 350connected in series. The magnetic field generator also has a ferritecore 360. The inductor 355, first capacitor 350 and ferrite core 360 maybe similar to the ones described in the embodiment shown in FIG. 3. Acontroller 315 controls the operation of the signal generator 310. Themagnetic field generator 302 includes a warning system 390, receiver317, and antenna 318 similar to those described with regard to FIG. 2.The magnetic field generator 302 also includes a movable shunt 365.

The magnetic field generator of FIG. 4 also includes a voltage dividercircuit 307. The voltage divider circuit 307 has two resistors 351 a and351 b connected in series with each other and in parallel to theinductor 355. One side of a diode 352 is connected between the tworesistors 351 a and 351 b. The other side of the diode 352 is connectedto a second capacitor 353 and to an input of the controller 315. Oneterminal of the second capacitor 353 is connected to the diode 352 andan input of the controller 315. A second terminal of the secondcapacitor 353 is connected to the inductor 355, one of the resistors 351and a second input to the controller 315. An output of the controller315 is connected to the signal generator 310. A power source 320 isconnected to the signal generator 310 and a second power source 321 isconnected to the controller 315. Alternatively, the signal generator 310and the controller 315 may share a single power source 320 or 321.

The voltage divider circuit 307 outputs a smaller voltage than theactual voltage across the inductor 355. The voltage output by thevoltage divider circuit 307 (the feedback voltage) is input to thecontroller 315. As described above, the size of the magnetic fieldproduced by the magnetic field generator 302 is related to the voltageacross the inductor 355. The feedback voltage output from the voltagedivider circuit 307 directly relates to the voltage across the inductor355. Therefore, the controller may accurately predict the size of themagnetic field according to the voltage output by the voltage dividercircuit 307.

In one embodiment, current from the voltage divider circuit 307 passesthrough the diode 352 into a second capacitor 353 that accumulatescharge. The accumulated charge may be read out by the controller 315 anddisplayed by the magnetic field generator 302. In this manner, thecontroller 315 may compute the voltage across the inductor 355. Thecontroller 315 may then compute the size of the magnetic field producedby the magnetic field generator 302 based upon the voltage across theinductor 355. A worker may then manipulate the shunt 365 in relation tothe inductor 355 and ferrite rod 360 to change the inductance of theinductor 355. Correspondingly, the resonance of the magnetic fieldgenerator 302 and the voltage across the inductor 355 will change aswell.

In one embodiment, the controller 315 displays a value indicative of theinductance of the inductor 355. For instance, the controller 315 mayilluminate a series of lights, such as LEDs, indicating whether theinductance is too low for the magnetic field generator 302 to achievemaximum resonance or whether the inductance is too high for the magneticfield generator 302 to achieve maximum resonance. In another embodiment,the controller may display whether the magnetic field generator 302 isbecoming more or less resonant in response to movement of the shunt 365.In an embodiment, the controller 315 may illuminate a light indicatingthat the magnetic field size produced by the generator 302 has droppedbelow a predetermined amount. The predetermined amount at which awarning light is illuminated may be adjustable. In an additionalembodiment, the controller 315 may adjust the output of the generator302 to maintain the magnetic field at the predetermined size and, ifunable to maintain the predetermined magnetic field size, the controller315 may cause the warning system 390 to indicate a failure.

In one embodiment, the shunt 365 is fixed in place once the shunt 365has been manipulated to achieve the desired resonance. The shunt may befixed in place by a fixing means such as an adhesive or fastener, suchas one or more bolts, screws, clips, or clamps. In one embodiment, theshunt may be fixed in place by an adhesive applied through perforatedholes in the shunt 365. In another embodiment, the shunt 365 may be heldin place by friction. Furthermore, the shunt 365 may be supported inrelation to the ferrite core 360 in any suitable manner.

In yet another embodiment, the controller 315 may control the signalgenerator 310 to change the current, voltage, or voltage pulse widthgenerated by the signal generator 310 in response to an input voltagereceived by the controller 315 from the voltage divider circuit 307. Forinstance, the controller 315 may control the signal generator 310 tooutput a current, voltage, or voltage pulse width that achieves aparticular voltage or current across the inductor 355. In oneembodiment, the particular current or voltage across the inductor 355may correspond to a desired magnetic field size produced by the magneticfield generator 302. In another embodiment, the controller 315 maycontrol the signal generator 310 in order to generate a constant voltageacross the inductor 355.

FIG. 5 is a view like FIG. 3 of a third embodiment of a magnetic fieldgenerator. The magnetic field generator 402 of FIG. 5 is similar to themagnetic field generator of FIG. 4 in that it includes a signalgenerator 410, power sources 420, 421, controller 415, warning system490, receiver 417, antenna 418 first capacitor 450, inductor 455,ferrite core 460, voltage divider circuit 407, resistors 451 a, 451 b,diode 452, and second capacitor 453. The magnetic filed generator 402also includes an automatically adjustable member 466 for controlling theposition of the shunt 465. The shunt 465 may be supported in relation tothe ferrite core 460 in any suitable manner. The automaticallyadjustable member 466 is physically connected to the shunt 465 andincludes a device to adjust the position of the shunt 465, such as alinear actuator, servo, hydraulic, electrically activated polymer, orother device. The automatically adjustable member 466 is also connectedto the controller 415. As described above, the controller 415 receives asignal from the voltage divider circuit 407 that is indicative of thevoltage across the inductor 455. The controller 315 may output a signalto the automatically adjustable member 466 that causes the automaticallyadjustable member 466 to move the shunt 465 in relation to the inductor455 and the ferrite core 460.

In one embodiment of the magnetic field generator of FIG. 5, thecontroller 415 controls the automatically adjustable member 466 to movethe shunt 465 to maximize the resonance of the magnetic field generator402. At a maximum resonance of the magnetic field generator 402, thecurrent across the inductor 455 is at a maximum value for a givenvoltage. In one embodiment, the controller 415 may adjust the positionof the shunt 465 in relation to the inductor 455 and ferrite core 460 inorder to maximize the current flowing through the inductor 455. Inanother embodiment, the controller 415 may control the automaticallyadjustable member 466 to move the shunt 465 to increase or decrease thevoltage across the inductor 455 and, correspondingly, the size of themagnetic field generated by the magnetic field generator 402. In variousembodiments, the automatic adjustment of the shunt may be performed atvarious time intervals, such as at startup of the magnetic fieldgenerator, or may be ongoing.

FIG. 6 shows a cutaway profile view and FIG. 7 shows a side view of aportion of another exemplary magnetic field generator in accordance withanother embodiment. The magnetic field generator 602 includes a housing680. Within the housing is a ferrite core 660 and an inductor. As can beseen in FIGS. 6 and 7, a partially cylindrical shunt 665 partiallysurrounds an exterior of the magnetic field generator housing 680. Theshunt 665 may be supported in relation to the ferrite core 660 in anysuitable manner. In one embodiment, the shunt 665 may be manually movedlongitudinally along the surface of the magnetic field generatorhousing. In another embodiment, an automatically adjustable means andcontroller may automatically move the shunt along the magnetic fieldgenerator housing 680.

In one embodiment, the ferrite core 660 may have a diameter ofapproximately 1-1.5 inches and length of approximately 10-12 inches. Inthe embodiment shown in FIGS. 6 and 7, the diameter of the magneticfield generator housing 680 is approximately 0.5 inches larger than thediameter of the ferrite core 660. Because mounting the magnetic fieldgenerator 602 on a solid metal surface may adversely affect the magneticfield generated by the magnetic field generator 602, in the embodimentshown in FIGS. 6 and 7, the magnetic field generator 602 in theembodiment shown in FIGS. 6A and 6B is mounted at least a distance equalto the length of the ferrite core 660 away from a solid metal surface.

FIG. 8 shows a cutaway profile view and FIG. 9 shows a side view of aportion of another exemplary magnetic field generator in accordance withanother embodiment. The magnetic field generator 702 has anexplosive-proof inner housing 782 at least partially surrounding theferrite core 760 and inductor. A shunt 765 at least partially surroundsthe explosive-proof inner housing 782, ferrite core 760 and inductor.The shunt 765 may be supported in relation to the ferrite core 760 inany suitable manner. The shunt 765 is completely surrounded by apolycarbonate outer housing 781. In one embodiment, a non-metallichousing support 789 is located between the magnetic field generator 702and the mounting surface 787. In the embodiment shown in FIGS. 8 and 9,the magnetic field generator 702 is attached to the mounting surface 787and non-metallic housing support 789 with U-bolts 788. However, themounting configuration shown in FIGS. 8 and 9 is merely one of manypossible configurations of mounting the magnetic field generator 702.The embodiment of FIGS. 8 and 9 is capable of “in-by” use (i.e. withinhazardous work zones within a prescribed distance of a mine-cuttingsurface) because of the use of an explosive-proof inner housing 782.

In-by use capabilities may be of particular importance in theunderground mining industry. For instance, in South Africa, a worker ormachine is considered to be in-by if they are within 180 meters of thecutting face of a mine. Any equipment operating within 180 meters of thecutting face must be explosive proof and flame proof. Locations beyond180 meters from the cutting face are considered to be “out-by” andgenerally have less stringent regulations for equipment. A thirdclassification zone exists for equipment operating exclusively aboveground. The embodiment of FIGS. 8 and 9 is suitable for use in in-bymining situations.

FIG. 10 shows another exemplary magnetic field generator in accordancewith another embodiment of the magnetic field generator. The magneticfield generator 802 has a polycarbonate housing 880 and a magnetic shunt865 partially surrounding the magnetic field generator 802. Forinstance, the magnetic shunt 865 may surround the magnetic fieldgenerator 802, on one, two, three sides or four sides. In oneembodiment, the magnetic shunt 865 may be slid longitudinally along thepolycarbonate housing 880 of the magnetic field generator 802. A screw,adhesive or other fastening device 867 may be used to hold the shunt 865in place relative to the polycarbonate housing 880 in between movementsof the shunt 865. The shunt 865 may be supported in relation to themagnetic field generator 802 in any suitable manner.

FIGS. 11-15 are diagrams of an exemplary method for calibrating aproximity detection system in accordance with an embodiment. It may benecessary upon installation or at other times to calibrate the proximitydetection system in order to properly define a distance at which apersonal alarm device 903 becomes dangerously close to the magneticfield generator 902, and more importantly, to the dangerous equipment onwhich the magnetic field generator 902 may be mounted. In oneembodiment, with reference to FIG. 11, the magnetic field generator 902is calibrated by placing a personal alarm device 903 at a distance atwhich a warning zone is to be defined for the magnetic field generator902 and equipment. The personal alarm device 903 searches for themagnetic field 904 produced by the magnetic field generator 902 andresponds with an RF signal once the magnetic field 904 is detected. Asshown in FIG. 12, if the magnetic field 904 is detected by the personalalarm device 903, the magnetic field generator 902 decreases the size ofthe magnetic field 904 until the personal alarm device 903 no longerdetects the presence of the magnetic field 904. As shown in FIG. 13,once the personal alarm device 903 no longer detects the presence of themagnetic field 904, the magnetic field generator 902 begins toincrementally increase the size of the magnetic field 904. As shown inFIG. 14, the size of the magnetic field 904 is incrementally increaseduntil the personal alarm device again detects the presence of themagnetic field 904 and responds to the magnetic field generator 902 withan RF signal 914.

With reference to FIG. 15, the controller within the magnetic fieldgenerator 902 records the voltage across the inductor at which themagnetic field 904 was first detected by the personal alarm device andmaintains the voltage across the inductor at this recorded level. Themagnetic field generator 902 may maintain a magnetic field 904 having aconstant size by holding the voltage across the inductor constant. Inone embodiment, the controller maintains a constant voltage across theinductor by adjusting the pulse width of the voltage generated by thesignal generator. In another embodiment, the controller adjusts thevoltage output of the signal generator. In still another embodiment, thecontroller adjusts the current output of the voltage generator. In yetanother embodiment, the controller automatically adjusts the position ofthe shunt. In other embodiments, combinations of the previousembodiments may be employed in order to keep the magnetic field 904 at aconstant size.

In another embodiment for calibrating a proximity detection system, apower level across the inductor of the magnetic field generator 902 isselected by the operator. In one embodiment, the power level may beautomatically set by the controller or some other means. Further, in oneembodiment, and in further reference to FIG. 4, the power level is setby software in the controller 315 to a level that is less than themaximum power level of the magnetic field generator 302. For example,the power level may be set at 90%-95% of the maximum power level, oreven lower if required. This may enable the controller 315 to adjust thepower output to a higher level if required for control of degradingoutput due to conditions differing from the factory or laboratory, asexplained earlier. Once a power level has been selected, the shunt 365may be adjusted on the generator 302 to achieve maximum power output. Ifmore than one magnetic field generator 902 is used, in order to achievea special field shape, the shunts may be adjusted differently on eachgenerator. The final size of the magnetic field 904 may then be checkedto assure that the magnetic field 904 is not excessively large for theworking environment, such that nuisance alarms could become a problem.If the magnetic field 904 produced by the magnetic field generator 902is determined to be too large, a personal alarm device 903 is placed atthe location where the magnetic field 904 boundary should be located andthe calibration sequence described above is activated at the controller315. The controller 315 may then reduce the size of the magnetic field904 and, subsequently, incrementally increase the size of the magneticfield 904 until the personal alarm device 903 indicates that themagnetic field 904 has reached the personal alarm device 903. Havingachieved the desired magnetic field 904 size, the controller 315 mayrecord the feedback voltage from the inductor 355 and control thevoltage across the inductor 355 to maintain the feedback voltage asrecorded. By this method, the magnetic field generator 902 may generatea magnetic field 904 that is maintained at the chosen size and shape,even with changing conditions, such as temperature, or degradation ofcomponents over time. Maintaining a fixed magnetic field 904 size andshape is very important to provide the required safety zone and to alsoprevent nuisance alarms that result from the use of excessively largesafety zones or zones not having the needed shape.

With reference to FIG. 17, as the two vehicles 1005, 1015 approach oneanother at an intersection 130, the personal alarm devices 1003, 1013 oneach vehicle 1005, 1015 detects the magnetic fields 1004, 1014 producedby the magnetic field generator 1002, 1012 mounted on the other vehicle1005, 1015. The personal alarm devices 1003, 1013 and/or the magneticfield generators 1002, 1012 issue warnings to the operators of eachvehicle 1005, 1015 warning the operators of their dangerous proximity.In one embodiment, only the personal alarm device 1003, 1013 or magneticfield generator 1002, 1012 issues a warning. In another embodiment, thepersonal alarm devices 1003, 1013 or magnetic field generators 1002,1012, may automatically slow or stop their respective vehicles 1005,1015. Because of the momentum with which construction vehicles typicallytravel, in one embodiment, the magnetic field 1004, 1014 produced by themagnetic field generator 1002, 1012 may be larger than typical magneticfields generated by the proximity detection system.

While embodiments have been described in detail in connection with theembodiments known at the time, it should be readily understood that theclaimed magnetic field generator is not limited to the disclosedembodiments. Rather, the embodiments can be modified to incorporate anynumber of variations, alterations, substitutions or equivalentarrangements not heretofore described. For example, many configurationsof the housing and mounting methods for the housing may be practiced.The magnetic field generator is not limited to the housingconfigurations herein described.

1. A magnetic field generator, comprising: a signal generator foroutputting a voltage, a magnetic field generating circuit for generatinga magnetic field, wherein the magnetic field generating circuitcomprises: a capacitor, and an inductor comprising a ferrite core, and ashunt movably supported relative to the inductor such that moving theshunt changes a value of inductance in the magnetic field generatingcircuit.
 2. The magnetic field generator of claim 1, wherein the shuntat least partially surrounds the ferrite core.
 3. The magnetic fieldmarker generator of claim 2, wherein the shunt is cylindrical.
 4. Themagnetic marker field generator of claim 1, wherein the shunt ismanually adjustable.
 5. The magnetic marker field generator of claim 1,wherein the shunt is automatically adjustable.
 6. The magnetic fieldgenerator of claim 1, wherein the shunt comprises aluminum.
 7. Themagnetic field generator of claim 1, wherein the shunt is positionedrelative to the inductor such that a resonance of the magnetic fieldgenerating circuit is at its maximum value for a given voltage.
 8. Themagnetic field generator of claim 1, wherein the shunt further comprisesa restraining means for fixing the position of the shunt relative to theinductor.
 9. The magnetic field marker generator of claim 8, wherein therestraining means is one of the group consisting of a fastener, anadhesive, a screw and a friction pad.
 10. The magnetic field generatorof claim 1, further comprising a controller for detecting at least onephysical property of the magnetic field generating circuit.
 11. Themagnetic field generator of claim 10, wherein the physical property isvoltage or current.
 12. The magnetic field generator of claim 10,further comprising a voltage divider affixed circuit across theinductor.
 13. The magnetic field generator of claim 10, furthercomprising an adjustable member for moving the shunt.
 14. The magneticfield generator of claim 13, wherein the controller manipulates theadjustable member so as to position the shunt relative to the inductorsuch that a resonance of the magnetic field generating circuit is at itsmaximum value.
 15. The magnetic field generator of claim 1, furthercomprising an explosion proof housing, wherein the magnetic fieldgenerator comprising the explosion proof housing satisfies safetystandards for in-by use.
 16. The magnetic field generator of claim 1,further comprising a housing surrounding the capacitor and inductor,wherein the shunt is movably affixed to an exterior surface of thehousing.
 17. A method for adjusting the resonance of a magnetic fieldgenerator, comprising: installing a magnetic field generator at alocation at which the magnetic field generator is to be used, themagnetic field generator comprising: a signal generator for outputting avoltage, a magnetic field generating circuit for generating a magneticfield, wherein the magnetic field generating circuit comprises: acapacitor, and an inductor comprising a ferrite core, and a shuntmovably supported relative to the inductor such that moving the shuntchanges a value of inductance in the magnetic field generating circuit,and moving the shunt relative to the inductor to increase the resonanceof the magnetic field generating circuit for a given voltage.
 18. Themethod of claim 17, further comprising adjusting an adjustable memberattached to the shunt to move the shunt with respect to the inductor.19. The method of claim 18, further comprising adjusting the adjustablemember to maximize the resonance of the magnetic field generator. 20.The method of claim 18, further comprising manually adjusting theadjustable member.
 21. The method of claim 18, further comprisingautomatically adjusting the adjustable member.
 22. The method of claim18, further comprising adjusting the adjustable member using acontroller.
 23. The method of claim 17, further comprising detecting achange in the voltage across the inductor and indicating the change inthe voltage across the inductor using an indicator.
 24. A proximitydetection system, comprising: a plurality of alarm devices for detectinga magnetic field, and a magnetic field generator comprising: a signalgenerator for outputting a voltage, a capacitor in electroniccommunication with the signal generator, an inductor in electroniccommunication with the capacitor, a ferrite core at least partiallysurrounded by the inductor, and a shunt movably supported relative tothe inductor such that moving the shunt changes a value of inductance inthe magnetic field generator.
 25. The proximity detection system ofclaim 24, further comprising a controller for automatically moving theshunt, the controller comprising: a means for causing movement of theshunt, and a computing device for generating a control signal to causethe means for causing movement of the shunt to manipulate the shunt. 26.The proximity detection system of claim 24, wherein the plurality ofalarm devices are designed to be worn by people.
 27. The proximitydetection system of claim 24, wherein the plurality of alarm devices aredesigned to be mounted on vehicles.
 28. A method for adjusting the rangeof a magnetic field generated by a magnetic field generator for aproximity detection system, comprising: installing a magnetic fieldgenerator at a location, positioning a personal alarm device at adistance from the magnetic field generator, generating a magnetic fieldusing the magnetic field generator, wherein the magnetic fieldencompasses the personal alarm device, detecting the magnetic field withthe personal alarm device, decreasing the magnetic field size until thepersonal alarm device cannot detect the magnetic field, incrementallyincreasing the magnetic field size until the personal alarm devicedetects the magnetic field, and setting the magnetic field size equal tothe magnetic field size at which the personal alarm device detected themagnetic field.
 29. A method for adjusting the range of a magnetic fieldgenerated by a magnetic field generator for a proximity detectionsystem, comprising: installing a magnetic field generator at a location,selecting a power level of the magnetic field generator less than themaximum power level of the magnetic field generator, generating amagnetic field by operating the magnetic field generator at the selectedpower level, adjusting the position of a shunt relative to the magneticfield generator to maximize the size of the magnetic field generated bythe magnetic field generator at the selected power level, positioning apersonal alarm device at a distance from the magnetic field generator,and adjusting the magnetic field generator to generate a magnetic fieldhaving a boundry located at approximately the distance of the personalalarm device.
 30. A proximity detection system comprising: a firstmagnetic field generator for generating a first oscillating magneticfield, wherein the first magnetic field generator is arranged on a firstvehicle, a second magnetic field generator for generating a secondoscillating magnetic field, wherein the second magnetic field generatoris arranged on a second vehicle, a first magnetic field detector capableof detecting the second oscillating magnetic field and providing avisual and/or audible warning to an operator of the first vehicle upondetecting the second oscillating magnetic field, wherein the firstmagnetic field detector is arranged on the first vehicle, and a secondmagnetic field detector capable of detecting the first oscillatingmagnetic field and providing a visual and/or audible warning to anoperator of the second vehicle upon detecting the first oscillatingmagnetic field, wherein the second magnetic field detector is arrangedon the second vehicle.